The invention relates to an apparatus for exposing a substrate disc to a laser beam, which has a laser beam objective lens disposed in a mounting for displacement by means of piezoelectric translators, and a slide transversely displaceable in a table by means of a radial drive, the slide having a substrate holder disposed on a spindle driven by a rotary drive.
For the manufacture of master discs by which optically readable information media, such as compact discs, are produced, it is necessary that pits corresponding to the digital data be placed by the laser beam in precisely defined positions on a spiral track of a master disc. For this purpose, in addition to a precisely controlled rotatory speed, a very precise movement of the slide relative to the objective is necessary. Furthermore, it is necessary in the manufacture of such master discs to form a defined focal point, i.e., a light spot, on the photoresist coating of the substrate, with a defined spot diameter. This is accomplished with fixedly set end lens only if the substrate--usually a glass plate in practice--has always the same thickness and is absolutely flat at its surface. As a rule, the substrate thicknesses vary in the nanometer range, while the irregularities are in the micron range. This results in different distances between the lens and the substrate surface and thus to a variation of the laser spot diameter on the substrate. To prevent this it is known to adjust the end lens of the laser beam objective automatically by means of a moving-coil drive in accord with the varying distances. It has been found, however, that such drives do not have a sufficient dynamic range, so that irregulatories cannot be compensated due to excessively slow control, and therefore spot errors occur. This is true especially when operating at comparatively high recording velocities. In such a system, then, only substrates having very flat surfaces can be used, which are expensive to produce. Even then, recording speeds above 3 are not possible.
DE-A-38 28 145 has disclosed an apparatus for laser beam exposure of a substrate disc, in which the laser beam objective is mounted at the free end of a swiveling scanner arm. For focus control, a piezotranslator is disposed on the scanner arm, by which the scanner arm is able to execute a swiveling movement varying with the distance of the laser beam objective from the substrate. An additional piezotranslator can swivel the to scanner arm in a plane at right angles thereto, which serves for the rapid track following readjustment or correction in the direction of the track.
It is a disadvantage of the known apparatus that the adjustments produced by the piezotranslators result in an arcuate movement of the laser beam objective. This results in inaccuracies which limit the performance of the apparatus.
U.S. Pat. No. 4,385,373 discloses an apparatus for laser beam exposure in which the laser beam objective is held in a frame of four memory-like piezo elements configured as flexural bodies. By the simultaneous actuation of all of the piezo elements, the latter can flex and thereby adjust the distance of the laser beam objective from the substrate.
The focal adjustment explained in this patent has the disadvantage that the varying flexing of the piezo elements results in a tilting of the laser beam objective lens. Since necessarily not all piezo elements have an absolute identical behavior, such tilting movements can never be entirely prevented, so that, again, the accuracy of the focal adjustment leaves something to be desired, although the four piezo elements would involve great cost in achieving rapid adjusting movements.